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Mercury thermometers provide accurate, repeatable temperature readings from -40 degrees Celsius (which is also -40 degrees Fahrenheit) to 650 degrees Celsius (1202 degrees Fahrenheit). The range is affected by the metal with which mercury is amalgamated. Materials, such as alcohol, cling to the inside of glass containers, which can influence repeated temperature readings. Mercury does not cling to glass, and it expands and contracts at a predictable rate. Environmental concerns about mercury are prompting manufacturers to look for substitute materials, but many mercury thermometers are still in use.

The Bulb

A swelling, or bulb, at the bottom of the thermometer stores the mercury used in calculating temperature readings. As temperatures increase, mercury moves out of the bulb’s reservoir. As temperatures decrease, the mercury has some place to go as it recedes back into the bulb. The bulb is typically made of glass, but some thermometers may have bulbs made of stainless steel instead.

The Stem and Capillary Bore

Magnified glass makes up the outside jacket of the thermometer, which is called the stem. A thin bore inside the stem gives mercury a route of travel as it expands and contracts. The thin bore is called a capillary. Magnified glass makes reading the mercury level easier. A scale scribed on the outside of the stem gives the temperature reading. Since mercury moves predictably at various temperatures, the scribed markings portray an accurate measure of the temperature at points along the capillary. Some thermometers are called partial immersion types, and these include a line engraved across the stem that marks the depth at which the thermometer should be immersed in a substance in order to get a good temperature reading.

Expansion Chamber

Gases and air inside the capillary must have a place to collect as mercury rises. The expansion chamber is a small widening of the capillary on the end opposite the bulb, and it provides the necessary space. Gases compress inside the expansion chamber until the mercury flows back down the capillary and into the bulb as temperatures fall.

Contraction Chamber

The contraction chamber slows the mercury’s descent down the capillary. The contraction chamber affords the person taking a temperature reading a little time to get an accurate reading after removing the thermometer from a heated source. Some thermometers also have a tight restriction just above the contraction chamber. This pinched point virtually stops the flow of mercury back into the bulb for an extended time. The person must shake the thermometer to get it to read below the temperature just taken.

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About the Author

Robert Korpella has been writing professionally since 2000. He is a certified Master Naturalist, regularly monitors stream water quality and is the editor of freshare.net, a site exploring the Ozarks outdoors. Korpella's work has appeared in a variety of publications. He holds a bachelor's degree from the University of Arkansas.